1. Field of the Invention
The present invention relates in general to a method for optically measuring an AC electric field or an AC voltage which produces the AC electric field, by utilizing a beam of light as a medium for the measurement and the Pockel's effect for modulating the light beam. The invention is also concerned with an optical voltage/electric-field sensor adapted to measure an AC electric field or voltage according to the above method.
2. Discussion of the Prior Art
In recent years, optical measurement utilizing modulation of light by the Pockel's effect has been drawing growing attention of the industries concerned with the art of power transmission and distribution, for measuring or determining an AC electric field strength or a voltage associated with power transmission and distribution lines. Such optical measurement utilizes a beam of light as a medium having high degrees of insulation and freedom from electromagnetic induction noises.
Conventional optical sensors for measuring the AC electric field (voltage) by utilizing the Pockel's effect have a light-emitting portion for emitting a light beam for the measurement, a sensing head having a polarizer, a Pockel's-effect element, and an analyzer which are disposed in series with each other, and a light-detecting portion for receiving the light beam emitted from the light-emitting portion and transmitted through the sensing head. The polarizer and the analyzer of the sensing head are respectively located upstream and downstream of the Pockel's-effect element in the direction of propagation of the light beam. According to this known optical measuring method, the light beam emitted by the light-emitting portion is modulated by an AC electric field applied to the Pockel's-effect element while being transmitted through the sensing head. The transmitted light beam is received by the light-detecting portion which produces an electric signal corresponding to the received light beam. From the electric signal are retrieved or selected a DC component (Edc), and a signal component (E.omega.) having the same angular frequency as that of the AC electric field applied to the Pockel's-effect element. Such an optical sensor is capable of determining the AC electric field applied to the Pockel's-effect element, or the AC voltage producing the AC electric field, based on the ratio (E.omega./Edc) of the signal component to the DC component.
The known optical measuring method as described above accurately works when optical fibers or other suitable guiding means are used so that the light-detecting portion receives only the measuring light beam which has been emitted from the light-emitting portion. However, where there exists background or ambient light, such as sunlight or illuminating light, in the optical path of the measuring light beam, the light-detecting portion may undesirably receive the ambient light in the optical path, together with the measuring light beam originating from the light-emitting portion, thereby causing errors in the measurement of the AC electric field (voltage).
The conventional optical electric-field sensor as described above is adapted to determine or calculate a strength of the AC electric field, based on the ratio (E.omega./Edc) of the signal component (E.omega.) and the DC component (Edc) which are both selected from the output signal of the light-detecting portion. As is apparent from the equation (10) which will be described later, the equation which is representative of the ratio (E.omega./Edc) includes, for example, a half-wave voltage V.pi. of the Pockel's-effect element, which has dependence on the ambient temperature of that element. Therefore, t.he conventional optical sensor suffers from measurement errors caused by the ambient temperature at the Pockel's-effect element. While various methods have been proposed for improved measuring accuracy, there has not been found any practically effective solution to this problem. That is, the sensors which have been proposed to solve the problem are likely to be large-sized and complicated in construction, and are therefore unsuitable for practical use.